This invention relates to improvements in electric power assisted steering systems.
It is known to provide electric power-assisted steering systems comprising a DC power supply, an inverter bridge including a plurality of bridge switches connected to a multi-phase electric motor, the inverter bridge being configured to provide power assistance to a steering system of a vehicle, a bridge driver circuit for providing control signals to the inverter bridge, and a DC link capacitor circuit interposed in the DC link between the rectifier of the DC power supply and the plurality of inverter bridges. The DC link capacitor circuit helps to smooth out transients that may be introduced during operation of the circuit, and otherwise smooth out ripple from the DC supply to the motor bridge.
The DC link capacitor circuit may include a single DC link capacitor or multiple DC link capacitors, which operate in order that the system is able to manage maximum ripple and survive high thermal stresses, for example during specific load profiles. High stress load profiles may include, for example, a parking cycle typical of the power profile required when a user is performing a parking manoeuvre. By using multiple DC link capacitors, the ripple current is shared between the capacitors—equally, in an ideal case—and thus there is less stress on each.
When one DC link capacitor in a set is faulty, or multiple capacitors are faulty, the remaining capacitors must compensate for this fault by taking on additional loading and thus stress. With additional stress on the remaining capacitors, each is more prone to developing faults itself. If each capacitor fails in turn, it is possible that the bridge circuit would at some point become incapable of driving enough current into the motor, the link voltage would become unstable, and the power-assistance to the steering system would be unable to function.
In power-assisted steering systems with dual-bridge architecture, whereby two lanes of an assistance system drive a dual-stator motor, there is more resistance to fault, due to one of the bridges being capable of operating the motor even if the other bridge were to fail. In normal operation, each bridge may use up to 100% of its possible power capability, although this is normally not the case in normal driving conditions, or at least not for prolonged periods of time. When one bridge fails, the other bridge may then be more likely to be required to use 100% or close to 100% of its possible power capability. In this situation, it is more important that a latent fault is recognised, due to the reliance on only one bridge to provide power assistance; it is important to know that the remaining bridge is capable of providing the desired assistance.